Honda had won the 125 cc World Championship in 1961, 1962, and 1965. By the mid-1960s two-stroke machinery from Suzuki and Yamaha had closed the gap significantly, and a new design was required. Engineer Shoichiro Irimajiri — the same engineer responsible for the concurrent 250 cc six-cylinder programme — led development. The same logic applied: where four-stroke engines fire every other crankshaft rotation against a two-stroke's every rotation, the answer was to rev far higher by distributing reciprocating mass across multiple cylinders.
Honda's 1965 season was difficult. Even with an updated four-cylinder, the 4RC146, the team's best results were seconds. At the Japanese Grand Prix finale Honda introduced a five-cylinder prototype, the RC148, but Taveri retired from the lead with an oil leak. The RC148 was effectively structured as two and a half 50 cc twins, with a bore and stroke of 33 mm by 29 mm, displacing 124 cc and producing 34 bhp at 20,000 rpm.
The RC149 was introduced for the full 1966 season with revised dimensions — a bore of 35.5 mm and stroke of 25.14 mm, matching those of Honda's concurrently developed 50 cc RC116. Output rose to 38 bhp at 20,500 rpm. Twin oil coolers were mounted in the fairing sides, and an oil temperature gauge was fitted. The engine's narrow powerband, beginning at 18,000 rpm, and its tendency to stall below 13,000 rpm made the RC149 demanding to ride.
The cylinder bank was split into two groups: three cylinders with a 120-degree crankshaft and two cylinders with a 180-degree crankshaft, the two crankshafts connected by gears. Camshaft drive ran from between cylinders 3 and 4. Crankcases were cast from magnesium with four-valve heads throughout. Five carburettors were fitted, each with five jets, mounted on rubber stubs of varying lengths as part of the tuning process. Transmission used an 8-speed gearbox with multi-plate dry clutch.
Taveri's main rivals in the 125 cc class were Phil Read and Bill Ivy on Yamahas. Taveri won five of the nine rounds to secure the championship. Ralph Bryans rode a second RC149, achieving six podium finishes and third place in the final standings. Mike Hailwood also joined the RC149 line-up for the Isle of Man round, but the race's three-hour weather delay required retuning of the carburettors — a sensitive operation — and Hailwood was the best-placed Honda finisher in sixth.
Honda won the constructors' championship. For 1967 the company elected to concentrate its resources on the larger 250 cc and 500 cc classes, withdrawing from the 50 cc and 125 cc championships entirely.
The RC149 represented the limit of Honda's small-capacity engineering ambition in the 1960s. A five-cylinder 125 cc racing engine producing 38 bhp at 20,500 rpm was an extraordinary technical achievement for the era, and the machine's sole championship season in 1966 demonstrated the approach's effectiveness even as the engine's narrow powerband and sensitivity made it difficult to exploit fully. The RC149 belongs to the same generation of engineering radicalism as Honda's concurrent six-cylinder 250 cc and 350 cc machines — all designed by Irimajiri, all champions, all retired when new FIM regulations closed off the multi-cylinder configuration path after 1967.